This invention relates to optical waveguides and more particularly to single-mode optical waveguide structures especially suited for evanescent coupling either to other waveguides or to any of several peripheral optical components used in the field of optical communications.
The principles of physical optics in the near infrared and visible light region of the electromagnetic spectrum and the application thereof to the field of optical communications are now well known. The physics of light wave transmission is explained on the basis that such waves exist as an infinite number of electromagnetic modes, each such mode having its own propagation and distribution characteristics. Because each mode of light traveling along an optical waveguide such as a glass fiber structure propagates at its own characteristic velocity, if the same information to be communicated is supplied to all modes at one end of the glass fiber, the information will be dispersed as a result of the multiple modes reaching the other end of the same fiber at different intervals of time. For this reason, the quantum of information capable of transmission along an optical waveguide is maximized when the information carrying light passing along the waveguide is restricted to a single mode.
Optical waveguides typically employ a central core of optically pure glass such as fused silica and a cladding of the same or similar glass differing only in that the glass of the cladding has a lower index of refraction than the core. The difference in the refractive indices of the core and cladding is achieved by doping either the core to increase its index of refraction or the cladding to reduce the refractive index thereof in relation to that of the core. To restrict the passage of light through such an optical waveguide to a single mode of electromagnetic energy, the diameter of the core, the wave length of light to be transmitted, and the difference between the indices of refraction in the core and cladding are interrelated. For practical single mode transmission fibers in the wavelength region between 0.8 and 1.6 microns, the core diameter must be kept in the range approximately one to five microns. Considering the diameter of a human hair is approximately fifty microns, the size of such single-mode fiber cores approaches microscopic dimensions. As a result, singlemode fibers present such problems to handling and coupling that in spite of recognized capacity, they were rejected in favor of larger, less efficient, multi-mode fiber transmission lines in the early stages of optical communications.
U.S. Pat. No. 4,315,666 issued Feb. 16, 1982 to the present inventor discloses several embodiments of rotationally non-symmetric optical waveguide fibers by which both handling and coupling problems are substantially overcome. The location of the waveguide core near one portion of the cladding periphery facilitates lateral coupling of one waveguide fiber to another by evanescence. In addition, the rotationally non-symmetric structure enables a relatively large cladding diameter for the extremely small core and thus provides an overall waveguide size which is readily handled and more easily coupled than prior single-mode waveguide structures. Prototypes embodying the fiber structures disclosed in this patent have demonstrated significant potential as a total solution to the problems associated with coupling single-mode fibers. On the other hand, there are coupling situations where the embodiments of this patent suffer some disadvantage. For example, in making a toroidal resonant cavity, using a D-shaped fiber, an axial twist of 180.degree. is required in order to achieve the overlap needed for lateral coupling to close the loop. Similarly, where a branch line of an optical waveguide system is connected to the trunk line by a linear resonant cavity or Fabry-Perot device, it is sometimes necessary for the relatively short length of the linear resonant cavity to be twisted for suitable lateral coupling of the cavity to both the trunk and the branch line of the waveguide system.
In light of the foregoing, there is a need, therefore, for a single-mode optical waveguide fiber which can accommodate lateral evanescent coupling at more than one peripheral area while retaining ease of handling in general.